Method of cooling and dehumidifying compressed gases



NOV. 2, 1954 M H OLSTAD ETAL METHOD OF COOLING AND DEHUMIDIFYINGCOMPRESSED GASES led Sept. 29, 1951 @Q A mw a United States Patent OCOOLING AND DEHUMIDIFYING COMPRESSED GASES vApplication September 2 9,1951, Serial No. 248,998 2 claims. (ci. 18s-*120) METHODv F Thisinvention lrelates to a method of cooling and dehumidifying compressedgases and more particularly where:i a relatively high degree ofdehumidiiication is require With many uses of compressed air, a high`degree of dehumidication is desirable, such as with so called instrumentair .where the compressed air is used for operating delicate pneumaticcontrol instruments the parts of which arey particularly susceptible todisadvantageous oXidation of parts in contact with the compressed airand where drops of condensate in the compressed air can put theinstrument out of order; also where the compressed air is used fordriving equipment having rotating parts where water in the compressedair supplied to drive the equipment washes out the lubricant and causesyrapid wear of the equipment; also where the compressed air is used inpaint spraying or metallic Vshot blast equipment where the presence ofwater in the compressed air causes the paint to blister or rusting ofthe shot; and also where the compressed .air is used under freezingconditions where the moisture in the compressed air tends to freeze onrelease thereby to freeze up the equipment being served and haltingproduction. 4

' Further, in various processes, particularly heat treating processes,it is desirable to effect a part of the processing in a chemically inertatmosphere to avoid undesired chemical reactions. For example, in theheat treating of certain metals, it is desirable to maintain an inertatmosphere in the heat treating furnace to avoid scaling or otherundesired surface change of the metal parts under treatment. Further inthe processing of oils it is desirable to effect a part of theprocessing in a 'chemically inert atmosphere. Such chemically inertatmospheres are usually maintained at higher than atmospheric pressureso that there is no danger of contamination through leakage, any leakagemerely resulting in the loss of a part of the chemically inert gas butnot resulting in contamination of the atmosphere.

It has been proposed, as set forth in the Olstad Patent 2,454,883 forApparatus for Cooling Compressed Gases, dated November 30, 1948, todehumidify compressed gases by the cooling thereof to a temperaturebelow its dewpoint temperature using surface Water or by subjecting thecompressed gases to the evaporative cooling effects of streams of airand water. Such cooling media, however, are not available atparticularly low temperatures and hence impose denite limitations on thedegree to 'which the compressed gas can be dehumidied by loweringy thedewpoint temperature thereof.

lt has also been proposed asset forth in the Kals Patent No. 2,568,891,granted Sept. 25, 1951, for Heat EX- change Apparatus to dehumidify gasunder pressure by passing it in Contact with a low temperature coolingcoil maintained substantially below the freezing point of water andspraying an antifreeze liquid over this coil to prey vent the frostingthereof. With such a method, as pointed out later in greater detail,while compressed gas of low dewpoint temperature and hence a high degreeof dryness can be obtained, refrigeration is required and also, where inthe practice of the present invention Vrefrigeration is also required toobtaina very low vdewpoint temperature, a very substantially greaterexpenditure of power for refrigeration is required with this priorprocess as compared with the practice of the present invention.

lt is accordingly one of the principal objects of the present inventionto cool and to dehumidify compressed 2,693,247 Patented Nov. 2j, 1954 2f gases to a relatively high degree of dryness without th necessity forrefrigeration.

It is another important object to dehumidify such gases to a stillhigher degree of dryness with less refrigeration than has beenheretofore necessary for such a result.

t Another object is to eliminate all problems of pressure balancing suchas usually arise in treating compressed gases.

Another object is to permit of continuous dehumidication of gases underpressure to a very low dewpoint so as to provide gas having a very lowmoisture content.

Another object is to provide such a process which can be practiced withapparatus which is simple in construction and operation and whichrequires little supervision.

Another object is to provide such a method which can be readily set tomaintain the exact degree of dehumidication desired.k

Another object, where refrigeration is used, is to eliminate the dangerof freezing of the condensate on the 10W temperature cooling surfaces.

Another object is to provide such apparatus which is free from thedanger of leakage, such being a particularly important objective wheredeadly gases, such as carbon monoxide, lare being handled.

Other objects and advantages of the invention will be apparent fromy thefollowing description and drawings in which: n

Fig. l is a schematic representation of apparatus for dehumidifyingcompressed gases in accordance with the present invention, the apparatusalso including a concentrator for maintaining the solution used toeffect dehumidiiication at the required strength.

n Fig. 2 is a horizontal section taken on line 2 2, Fig. l.

lln the following description it will be assumed that the compressed gasto be dehumidified will be compressed air vfrom a compressor having anaftercooler in which a part of the heat of compression has been removed,the apparatus forming the subject of the invention operating both todehumidify air and also to further reduce the temperature of thecompressed air. In general, the apparatus shown comprises a dehydrator,indicated generally at'5,-and a concentrator, indicated generally at 6,the solution used as the dehydrating medium in the dehydrator beingrecirculated through the concentrator so as to maintain its desiredstrength and effectiveness.

Referring more particularly to the dehydrator, this dehydrator is shownas comprising a vertical cylindrical sheet metal casing 8 havingoutwardly dished upper and lower end heads 9 and 10 and supported onlegs 11. The lower end head 10 forms a sump and is shown as providedwith the usual drain 12.

As best shown in Fig. 2, the compressed air is supplied to the apparatusfrom a pipe 13 which can be connected directly with an air compressoralthough preferably the compressed air is precooled before beingadmitted to the dehydrator by passage through the usual aftercooler ofthe compressor. Neither the compressor nor its aftercooler is shown. Thecompressed air from the pipe 13 is admitted to the casing 8 through atangential inlet 14, this tangential inlet serving to initially directthe compressed air to rotate in the casing 8 in a counterclockwisedirection as viewed in Fig. 2. This air forms a column moving helicallyupward through the casing and leaves through an outlet pipe 15 whichextends radially from the wall of the casing 8 and is located near theupper end head 9.

An L-shaped baille 16 is secured in the casing 8 in advance of theoutlet pipe 15, the purpose of this baille being to reduce the amount ofentrained solution carried out of the casing 8 with thek compressed air.

The compressed air then passes through a liquid-fromair separator 18which can be of any conventional form. As shown, this separatorcomprises a cylindrical shell 19 connected at its upper end with thedehydrator compressed air outlet pipe 15, the air initially passingaround an outlet 20 having a downturned end arranged axially of theshell 19. This outlet 20 communicates with an outlet neck 21 whichconnects with the pipe used to distribute the dehydrated compressed air.The bottom of the separator 18 is shown as being dished and as having aninternal bridge-like baille 22 over a solution drain outlet 23. Thesolution from this outlet flows through a pipe 24 to the bottom of thecasing 5.

The solution used to dehumidify the compressed air passing through thecasing 8 is preferably an aqueous solution of an organic substancehaving a boiling point higher than the normal boiling point of water,examples of such substances being the polyhydroxy organic compounds,such as triethylene glycol. However, other hygroscopic substances, sucnas lithium chloride can also be used. In accordance with the presentinvention, a body 25 of such solution is maintained in the sump orbottom 10 of the casing 8 and is continuously recirculated through aheat exchanger 26 and a spray tree 28.

For this purpose a solution outlet line 29 connects the bottom of thecasing 8 with the inlet of a recirculating pump 30 continuously drivenby an electric motor 31. The outlet line 32 from the pump connects withone inlet of the heat exchanger 26 which can be of the shelland-tubetype. In dehydrating compressed air coolant is supplied to this heatexchanger Z6 from an inlet pipe 33, this coolant passing in heatexchange relation with the solution from the pump 30 and leaving througha coolant outlet pipe 34. The outlet line 35 for the recirculatingsolution from the heat exchanger 26 extends through the side wall of thecasing S near its upper end and thence downwardly along the axis of theshell 8 and is capped, as indicated at 36, at its lower end. Thisaxially extending part of the pipe 35 is provided with a plurality ofradiallyextending branch pipes 38 shown as arranged in vertical spacedgroups each having one or more spray nozzles 39 through which thelrecirculated solution is discharged into the helically moving, risingcolumn of air in the casing 8, and to assist the helical movement of therising column of air each nozzle is directed horizontally to project ina counterclockwise direction as viewed in Fig. 2.

The solution so recirculated abstracts moisture from the compressed airand accordingly is diluted as it is used. In order to maintain thestrength of the solution it is necessary to concentrate it byevaporating water from it so as to maintain a high concentration of thesubstance used as the dehydrating agent.

To this end a solution outlet line 40 connects with the bottom of thecasing 8 below the level of the liquid in this casing, the solutionbeing shown as being moved through this line by the pressure of thecompressed air passing through the shell 8. This solution then passesthrough a heat exchanger 41 and thence through a pipe 42 to the casing43 of the concentrator 6. The` line 42 is shown as having a valve 44 tocontrol the rate of feed of the dilute solution to the concentrator 6.

The concentrator casing is shown as being of rectangular form inhorizontal section 'and as having a central vertical partition 45extending downwardly from this top wall 46 so as to provide a downowpass 48and an upflow pass 49, the partition 4S being spaced from thebottom of the casing to permit air to jowfrom the bottom of the downflowpass 48 over to the bottom of the upflow pass. This bottom of the casing43 is shown as comprising an inclined part 50l under the upow pass 49and draining into a sump 51 under the downflow pass 48, this sumpcontaining a body 52 of the solution which is recirculated through theconcentrator. This recirculation is through a pipe 53 leading from thebottom of the sump 51 to a recirculatin'g pump 54. The outlet line 55from this pump is shown as extending through the side wall of thedownflow pass 48 of the. casing 43 and as terminating in a spray tree56. The spray tree is shown as comprising a plurality of horizontalbranches 58 carrying downwardly directed nozzles 59 which discharge thesolution downwardly'nto the air in the downflow pass 48.

Fresh air is preferably used for concentrating the solution, this freshair entering the upper end of the downflow pass 48 through an inlet 60.This air flows downwardly past the sprays issuing from the nozzles 59and past a heater conventionally indicated at 61. This heater can besupplied with a heating medium from an inlet 62, the heating mediumleaving at 63.

The air from the downflow pass 48 passes under the central partition 45and flows up the upow pass 49. This air passes a reflux condenser whichis shown conventionally as being a cooling coil 64, the coolant enteringat and leaving at 66. From the reflux condenser, the air is shown aspassing through a plurality of eliminator plates 68 the purpose of whichis to whip the air back and forth so as to remove entrained solutiontherefrom. The air leaves the upow pass through an exhaust outlet 69from which it is discharged back to the outer atmosphere.

In this recirculation from the nozzles 59 of the spray tree 56 againstthe heater 61 and into the stream of fresh air passing down the downflowpass 48 mixed vapors of water and the organic substance are transferredto the stream of air, this transfer being principally of water vapor sothat the solution so sprayed into the air stream is continuously beingreconcentrated. The reconcentrated solution leaves the body of solution52 through an outlet pipe 70 connected with the inlet of a return pump71. The outlet 72 of this pump connects with the heat exchanger 41 inwhich it passes in heat exchange relation with the cool dilute solutionentering the concentrator 6. From the heat exchanger 41 the concentratedsolution passes through a pipe 73 to the body 25 of solution maintainedin the sump 10 of the dehydrator 5.

As an example ofthe operation of the apparatus in obtaining a highdegree of dehumidication of, say, compressed air without the use ofmechanical refrigeration, it will be assumed that the apparatus is tohandle 2700 cubic feet per minute of free air having a dry bulbtemperature of 64 F. and saturated with moisture. It will also beassumed that this free air is compressed to 110 e pounds gage pressureand that it is necessary to dehumidify this compressed air to have adewpoint temperature of 55 F.

This could not be done without mechanical refrigeration with any of theprior art processes mentioned. Thus surface water is not available inthe summertime at temperatures below 55 F. to cool the compressed air tothis temperature and establish its dewpoint temperature at the required55 F. Also since it is assumed that the outside air is saturated,evaporative cooling would be ineective and'in any event could not beused to achieve a temperature below the assumed 64 F. since this is alsothe wet bulb temperature ot' the outside air.`

With the present appartus, using as the spray liquid an aqueous solutionof a polyhydroxy compound having hygroscopic properties, such astriethylene glycol, and at a concentration in the order of 92%, it isonly necessary to maintain this solution at a temperature of 98 F. tobring the `dewpoint temperature of the compressed air to the 'required55 F.` the compressed air being reduced in temperature to 99 F. byContact with the spray solution. This can obviously be accomplishedwithout refrigeration since the temperature of the spray liquid can beeasily maintained at 98 F. with a coolant having a temperature of 70 F.and for which surface water or evaporative cooling as described in saidOlstad patent l 2,454,883 can be used.

Thus, under the assumed conditions, 2700 C. F. M. of free air at 64 F.saturated with moisture is compressed to lbs. gage pressure and admittedthrough the tangential inlet 14 of the spray chamber 5. This compressedair, after passing through the after cooler of the compressor, can beassumed to have a temperature of 108 F. This compressed air, enteringthe spray chamber 5 tangent-ially, moves upwardly therein in a helicalpath to provide an upwardly moving rotating co1- umn, this helicalmovement being assisted by the projection of the sprays from the nozzles39 which are spraying the hygroscopic solution being recirculated by thepump 30 through the heat exchanger 26, spray tree 28,

y spray nozzles 39, sump 10 and back to the pump 30.

The compressed air leaves the spray chamber 5 through its outlet 15 at atemperature of 99 F. and at a dewpoint temperature of 55 F., thesensible cooling of from 108 F. to 99 F., representing 103,700 B. t. u.per hour being effected by maintaining the temperature of the sprayliquid 25 at 98 F. and the moisture removal to provide the low vdewpointtemperature being effected by absorptlon in to the hygroscopic sprayliquid. The leaving compressed 'air passes through the liquid-from-airseparator 18 in which entrained liquid is removed and. returnedthroughthe line -24 to the body 25m the sump 10 of the spray chamber 5.

This sensible heatof 103,700 B. t. u. per hour is removed frorn thelspray solution by passing it through the heat exchanger 26 to which, asindicated, coolant is suppliedat 70 F. to restorevthe temperature'of thelsprays from the nozzles 39 to 98 F. As indicated, cooling liquid atthis relatively high temperature of 70 F. can n readily be suppliedwithout refrigeration. f, 4

` In contrast if the assumed 2700 cubit feet of free air per minutesaturated at a temperature of 64 F. and com-l pressed to 110 pounds gagepressure and aftercooled 'on leaving the compressor to 105 F. were to bedehumidied by lowering its temperature to its assumed 55 F. dewpointtemperature, as described in the said Kals patent, it would be necessaryto remove 157,000 B. t. u. per hour of both the sensible and latent heatfrom the compressed gas and to obtain cooling to the necessary 55 F. dryand wet bulb temperature would require a coolant temperature of notsubstantially more than 40 F. Necessarily, therefore, refrigerationwould be required to obtain the 55 F. dew point temperature of thecompressed air whereas in the practice of the present invention thenecessity for such refrigeration is avoided and at the same time exactcontrol of the dewpoint temperature of the leaving compressed air isobtained.

. The moisture so absorbed into the spray water 25 must be continuouslyremoved in order to maintain it at the assumed concentration of 92%. Toso maintain this concentration, spray solution from the body 25 in thesump of the spray chamber is continuously discharged through line 40,and heat exchanger 41 to the body 52 in the sump 51 of the concentrator6. This body of liquid is at the assumed concentration of 92% and ismaintained at this concentration by recirculation by the pump 54 throughthe spray nozzles 59. The downward sprays from these nozzles induces arelatively slow flow of fresh air from the fresh air inlet 60 throughthe downpass 48, uppass 49 and out through the outlet 69. This spraywater is also heated by repeated contact with the heating coils 61 andtransfers this heat to the air stream so as to increase itsabsorbtivity. As a result the air is substantially saturated at anelevated temperature and because of the higher boiling point of theorganic substance in the solution and because of its substantiallycomplete saturation with water vapor very little of the organicsubstance is evaporated. The spray liquid is accordingly concentratedand after passing the heating coil 61 falls back to the body of liquid52. The dilute liquid from the liquid body 25 in the spray chamber 5 soowing to the liquid body 52 in the concentrator 6 is replaced byconcentrated liquid moved by the pump 71 from the liquid body 52 to theliquid body 25.

Further to minimize loss of the substance in the solution which rendersit hygroscopic, the air owing up the upflow pass 49 of the concentrator6 encounters the cooling coil 64 which acts as a reflux condenser. Thecooling of the air causes it to become saturated and also condense someof its moisture. Because of the differences in absorbtivity, thecondensate from the reflux condenser 64 is composed largely of the smallamount of hygroscopic substance vaporized into the air by the spraynozzles 39. This recovered substance flows back to the sump 51 to rejointhe body 52 of solution.

As a second example of the operation of the apparatus, it will beassumed that it is necessary to have compressed air with a dewpointtemperature of 20 F. as compared with the previous example where therequired dewpoint temperature of the leaving compressed air was 55 F.With such a required dewpoint temperature of 20" F. the dry bulbtemperature of the leaving compressed air would be 43 F. Otherwise itwill be assumed that the same conditions obtain, that is, that theapparatus is to handle 2700 cubic feet per minute of free air saturatedat 64 F. and that this air is compressed to 110 pounds gage pressure andleaves the aftercooler of the compressor saturated at 108 F.

On this second example refrigeration is required, but, as pointed outlater, much less refrigeration is required as compared with a svstemwherein such dehumidication is obtained by cooling the compressed air tothe required dewpoint temperature of 20 F.

With such a required leaving dewpoint temperature, the spray solutionrecirculated by the pump 30 from the liquid body 25, heat exchanger 26,spray tree 28 and its nozzles 39 back to the liquid body 25 would bemaintained at a temperature of 40 F. and would have the same assumedconcentration of 92%.

The compressed air at the assumed 110 pounds gage pressure and at 108 F.entering through the tangential inlet 14 is brought by contact with thesprays from the nozzles 39 to a dry bulb temperature of 43 F. and to adewpoint temperature of 20 F; asi required. The maintaining of therequired 40 F. temperature of the body 25 of'spray solution isachieved'by its passage through the` heat 'exchanger 26 in heat exchangewith a coolant cooled by mechanical refrigerationl and the concentrationof. this. spray solution is maintained by recirculation throughthelconcentrator 6 asv previously described. f

While mechanical 'refrigeration is 4required to bring thecompressedfair'to' this assumed 20 F. dewpoint temperature, much lessrefrigeration is required than if the same dewpoint temperature wereobtained by cooling this compressed air to this temperature as describedin the said Kals patent. Thus the present process as above describedrequires the removal of 880,500 B. t. u. per hour with 30 F.refrigeration to maintain the coolant at the said 40 F. temperature.This involves the expenditure of 73 horsepower per hour. To cool thesame compressed air to the required dewpoint temperature of 20 F. wouldinvolve the removal of 940,000 B. t. u. per hour with a refrigerant at 5F. and with the expenditure of 156 horsepower per hour.

It will accordingly be seen that the present invention provides a methodof dehumidifying compressed gases to a high degree of dryness with theexpenditure of little if any refrigeration and in which the dewpointtemperature of the leaving air can be accurately controlled. It willfurther be seen that there is no problem of balancing pressures. It willfurther be seen that while solutions of organic hygroscopic substances,such as polyhydroxy compounds, are preferred, the invention can bepracticed with solutions of inorganic hygroscopic substances, such asaqueous lithium chloride solutions.

We claim:

l. The method of dehydrating compressed gas which comprises sprayinginto a stream of said compressed gas a multiplicity of sprays of anaqueous solution having a hygroscopic substance the boiling point ofwhich substance is higher than the normal boiling point of water,withdrawing a part of said solution and passing it in indirect heatexchange relation with a coolant to abstract heat from said solution,returning said withdrawn part to said sprays, withdrawing another partof said solution, heating said another part of said solution to a degreethat the heat will vaporize most of the water but not affect thehygroscopic substance, contacting an extended surface of said heatedsolution with a stream of air to transfer from said heated solution tosaid stream of air mixed vapors of said water and substance whereby saidheated solution is concentrated with respect to said substance,thereafter completely saturating the air with water vapor to condensesubstantially all the hygroscopic substance from the air in the form ofan aqueous solution thereof, and returning said concentrated part ofsaid solution for repeated contact with said compressed gas.

2. The method of dehydrating compressed gas which comprises sprayinginto a stream of said compressed gas a multiplicity of sprays of anaqueous solution having an organic hygroscopic substance the boilingpoint of which substance is higher than the normal boiling point ofwater, withdrawing a part of said solution and passing it in indirectheat exchange relation with a coolant to abstract heat from saidsolution, returning said withdrawn part to said sprays, withdrawinganother part of said solution, heating said another part of saidsolution to a degree that the heat will vaporize most of the water butnot affect the hygroscopic substance, spraying said heated solution intoa stream of fresh air to transfer from said heated solution to saidstream of fresh air mixed vapors of said water and organic substancewhereby said heated solution is concentrated with respect to saidorganic substance, cooling said stream of fresh air after contactwith'said heated solution to a temperature below the dewpointtemperature thereof to effect reflux condensation of said organicsubstance and returning said concentrated part of said solution forrepeated contact with said compressed gas.

References Cited in the le of this patent UNTTED STATES PATENTS NumberName Date 1,905,068 Speer Apr. 25, 1933 2,017,027 Forrest Oct. 8, 1935(Other references on following page) Number 7 UNITED STATES PATENTSOTHER REFERENCES Publication, The Dehydration of High Pressure NaturalGas, by Allyne, in Gas Age Record, May 18, 1935, 5 page 493-8.

